Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and devices used for marine seismic surveys, and, more particularly, to methods and devices used for firing marine seismic sources.
Discussion of the Background
Offshore drilling is an expensive process. Therefore, those undertaking offshore drilling use a profile (image) of the geophysical structure under the seafloor obtained by seismic surveys to avoid a dry well. However, due to the high cost of marine seismic surveys, there is continuous interest in decreasing their duration (i.e., increasing survey's productivity), while also increasing (or at least maintaining) data quality.
In a marine seismic survey, a vessel tows one or more sources that generate seismic waves. The seismic waves travel through the water and then penetrate the geophysical structure under the seafloor. The waves are reflected at interfaces between layers of the geophysical structure under the seafloor, with the layers differentiated by the different speeds at which the seismic waves propagate through them (i.e., a discontinuity occurs in the propagation speed as a function of depth at a layer interface). The reflected waves carrying information about layer characteristics and the location of layer interfaces are detected by seismic receivers.
Incident waves are P-waves (also known as pressure, longitudinal or primary waves). At a solid-solid interface (i.e., an interface between subsurface layers through which seismic waves propagate at different speeds), both P-waves and S-waves (also known as shear, transversal or secondary waves) may emerge as a consequence of incident wave's reflection and refraction. Reflected S- and P-waves carry complementary information. Generating an S-wave-based image in addition to the P-wave-based image gives access to an additional rock parameter (e.g., Poisson's ratio or simply the velocity ratio Vp/Vs), which enables better discrimination of the layers than P-wave-based imaging alone (e.g., allows better distinction of layers' porosity).
Since S-waves do not propagate through the water, seismic receivers are placed on the seafloor to detect both reflected P- and S-waves. Ocean bottom multi-component sensors (OBS 4C) are used in marine seismic surveys to detect both reflected P- and S-waves. For example, such an OBS 4C sensor may include a hydrophone and a three-component (3C) geophone, or a three-component (3D) accelerometer.
From the standpoint of generating incident seismic waves, marine surveys in which OBS 4C detect the seismic reflections may:                (1) use a single vessel towing a single source and achieving a productivity P;        (2) use a single vessel towing a dual source, including two sub-arrays triggered in flip-flop mode (i.e., alternating), achieving a productivity 2P; or        (3) use dual vessels, each towing a single source triggered in a radio-synchronized flip-flop mode, achieving a productivity (1+x)P, where x is the fraction of time the two vessels are online and shooting.        
FIG. 1 schematically illustrates a marine survey system (bird's eye view) including a vessel 10 towing, along a trajectory line 15, a dual source 20 including sub-arrays 22 and 24. A controller C (which may be located on vessel 10) controls alternatively firing sub-arrays 22 and 24. A few shot locations 30, 32, 34 and 36 are illustrated in FIG. 1. A corresponding time line 50 is illustrated parallel to trajectory line 15. Along time line 50, time runs from the bottom to the upper part of the line. Rectangles 1, 2, 3 and 4 on time line 50 represent recording times during which a data acquisition system records data related to S- and P-wave reflections detected by seismic receivers following shots at locations 30, 32, 34 and 36, respectively. Conventionally, a shot is fired only after all the data pertaining to reflections from a previous shot have been recorded. Thus, rectangles 1, 2, 3 and 4 do not overlap.
FIG. 2 is a graph illustrating time distribution of data corresponding to two records following a pair of successive shots. The x-axis of the graph represents the seismic receiver offsets, which determine distances from the shot location to respective seismic receivers. The y-axis is the time. Unlike in FIG. 1, in this graph, time flows downward from the top. The continuous arched lines in this graph correspond to P-wave reflections, and the dashed arched lines correspond to S-wave reflections. Data representative of reflections from the same interface has a curved profile on this graph because the farther the seismic receivers are from the shot location, the longer paths the incident wave and reflected wave(s) travel to and from the reflecting interface. The longer paths cause delays in detecting reflections by distant seismic receivers compared to the seismic receiver close to the shot location. The deeper the reflecting interface is, the more pronounced the curvature, while reflected waves reach a larger number and more distant seismic receivers.
Data acquisition is configured to record only information related to reflected waves detected by seismic receivers within a predetermined distance (e.g., an offset range of ±6,000 m) from a shot location (which is located at “0” on the x-axis).
Waves traveling through the solid layers are also absorbed and dispersed (besides being reflected). Therefore, a predetermined incident wave can be used to explore a limited depth. As a consequence, listening time following a shot is also limited, e.g., to a few seconds. Moreover, with S-waves traveling about twice as slow through solid layers as P-waves, P-wave listening time (PLT) is shorter than S-wave listening time (SLT). During PLT, a record related to a shot includes data related to both reflected P- and S-waves. After the end of PLT until the end of SLT, the record includes only data related to reflected S-waves. PLT and SLT are defined relative to the seismic receivers within a predetermined distance from the shot location. The last reflected wave 214 in a record related to a shot is an S-wave, and it is also known as a “horizon.”
Conventionally, a new shot is fired only after detecting and recording all the reflections from a previous shot. Rectangle 210 in FIG. 2 includes data related to reflections from first shot 212, and rectangle 220 includes data related to reflections from second shot 222. Rectangles 210 and 220 do not overlap.
Thus, by the time the second shot is fired, all seismic receivers within the predetermined distance left and right of the shot location have detected the last S-wave 214 reflected from the first shot. The length of rectangle 210 along the y-axis represents record time length (RTL), which may be even longer than SLT.
Productivity P is limited by the seismic record length (including dead-time when necessary), the distance between the shot locations (also called “source point interval”, SPI), and the requirement for 4D accuracy (i.e., precisely reproducing the shot locations for repeated seismic surveys of the same area).
FIG. 3 is a table illustrating theoretical calculations of the maximum towing speed for various seismic record lengths and different SPIs for different previously-discussed conventional marine survey methods. The values of the towing speed have to be greater than 3.5 kts (to maintain maneuverability of the source, where 1 kts=0.514 m/s), but not greater than 8 kts (due to constraints on the towed gear). Thus, if the calculated maximum speed is lower than 3.5 kts, no value is shown in the table, and if the maximum speed is higher than 8 kts, then this (8) maximum speed value is used.
Recently, there is a tendency to increase the record length from 6-8 s to more than 10 s, due to both recording more sensors covering a larger area and to recording slower S-waves in addition to P-waves. The SPI is typically in a range of 10-50 m, but increased density is desirable. The requirement for 4D accuracy has become more stringent, with tolerances of 5-10 m for 90% of the shot points, relative to referenced (intended) positions.
These constraints make dual-vessel flip-flop methods less attractive. Accordingly, it would be desirable to provide systems and methods for marine seismic surveys with higher productivity.